1. Field of the Invention
The present invention relates to an optical modulator which is used for a light control element such as modulation of a light wave and switching of an optical path, in particular, to an optical modulator which comprises a recess in a substrate forming the optical modulator, and a control electrode formed on said recess for controlling the phase of light.
2. Related Art Statement
Optical communication systems have attracted attention corresponding to an increase in the demand for high speed, large capacity data communication systems recently. Particularly, the optical modulator where an optical waveguide is formed in the substrate using a material having an electrooptic effect is widely known as a light control element suitable for optical broadband communication systems.
Also, as an optical modulator, which corresponds to a dense wavelength division multiplexing (DWDM) technology and high speed communication technology, the optical modulator which modulates a light from CW (Continuous Wave) laser using a Mach-Zehnder Interferometer (MZI) type external optical modulator (hereinafter described as LN optical modulator) using a material having the electrooptic effect, such as lithium niobate (LN), have been put to practical use.
As shown in FIG. 1, the MZI type external optical modulator is configured to divide a light inputted from an input optical waveguide 2 in half with a Y-shaped branching optical waveguide 3, to combine the lights guided through two optical waveguide 4 (the optical waveguides, wherein a light guided inside receives a phase control by the electrooptic effect of an electric field formed by a modulating electrode and a grounding electrode, which are control electrodes; hereinafter described as an optical waveguide modulation part) with another Y-shaped branching optical waveguide 5, and to put the modulated light to the exterior via an output optical waveguide 6. Each of these optical waveguides is formed by thermal diffusion of a high refractive index material such as Ti, on the surface of a substrate 1 having the electrooptic effect. In FIG. 1, the control electrodes such as the modulating electrode and the grounding electrode formed on the substrate 1 are omitted in order to make the shape of the optical waveguides easily understood.
In recent years, for the optical modulator having such optical waveguides, Japanese Patent Application No. H10-90638, for example, discloses an optical waveguide with ridge structure. In order to perform effective electric field effect of the control electrodes (a modulating electrode 21, and grounding electrodes 20 and 22) on the optical waveguide 4, this proposes a configuration as shown in FIG. 2 for providing ridges by forming recesses 10, 11 and 12 on both sides of the optical waveguide 4 and thereby allowing the electric field formed by the control electrodes to work intensively on the optical waveguide.
FIG. 2 is a cross-section view of the MZI type optical modulator shown in FIG. 1 along the dashed line A in a direction perpendicular to the optical waveguide modulation part 4. 7 in FIG. 2 indicates a buffer layer formed from SiO2 or the like.
However, in the optical modulator shown in FIG. 2, the grounding electrode 22 is formed over the recess 12. Thus, the temperature mainly around the control electrode section like the grounding electrode rises due to driving of the optical modulator over a long time and an increasing driving power that is inputted into the optical modulator. The difference in a coefficient of thermal expansion between the substrate forming the recess 12 and the grounding electrode formed on the recess 12 concentrates stress by the grounding electrode centrally in the side of the recess. Said stress generates stress-strain in the substrate. Since such stress-strain changes the refractive index of the substrate including the optical waveguide, it becomes a big obstacle in stably controlling the phase of light passing through the optical waveguide in the optical modulator.
Additionally, besides the above-described MZI type optical modulator, such stress-strain due to concentration of stress is also generated when a recess is formed in the surface of a substrate which forms an optical modulator and an electrode such as a grounding electrode is formed over said recess. Especially when an optical waveguide is formed adjacent to the recess, the influence of the stress-strain on optical modulation control becomes prominent.
The present invention intends to solve the above-described problems, to relax the stress generated in the recess formed in the surface of the substrate in the optical modulator, to inhibit deterioration of optical modulation control caused by the stress-strain in the substrate including the optical waveguide, and to provide the optical modulator which realizes stabilized driving over a long time.